1. Field of the Invention
This application is a continuation-in-part application of the application Ser. No. 07/375,162 filed Jul. 3, 1989 now abandoned.
This invention relates to thermal ink jet printing devices and, more particularly, to improved printheads which are maintained at a constant operating temperature so that droplet or pixel size does not vary with temperature.
2. Description of the Prior Art
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electrical pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
Thus, thermal ink jet devices operate by pulsing heating elements in contact with ink so that bubbles are nucleated, ejecting ink droplets toward the paper. It has been found during print tests that print quality is affected as the device heats up. In particular, if the device heats up too high (e.g., during extended high density printing), then it tends to lose prime, and one or more ink channels of the printhead cease to expel droplets. A less catastrophic defect, but still one that degrades print quality, is the increase in printed spot or pixel size as a function of device temperature. Through study of this phenomenon, it has been found that both the mass and velocity of the droplet increase with device temperature and that both the mass and velocity contribute to increased pixel size on the paper. For the carriage type ink jet printer with sufficiently high printing density, the spot size increases as the carriage traverses the page. Then as it pauses at the end of travel and reverses direction, it cools slightly, so that the next line or swath printed on the way back has increasing pixel sizes in the opposite direction. This gives rise to light and dark bands, which are most pronounced at the edges of the paper. Similarly, other patterns of high and low density printing are degraded by the increase in pixel size with device temperature.
Many of the prior art devices incorporate a heat sink of sufficient thermal mass and of low enough thermal resistance that the device temperature does not rise excessively. For one example of a thermal ink jet printhead having a heat sink, refer to U.S. Pat. No. 4,831,390 to Deshpande et al. This approach has eliminated the catastrophic printing failure mode. However, to lower the thermal resistance to the heat sink sufficiently that there is no appreciable device temperature rise in the time scale of a carriage translation in one direction across the paper, it may be necessary to take packaging approaches which would increase the cost or otherwise constrain the printer design in an undesirable way. The temperature rise must be maintained such that negligible image degradation occurs because of thermally induced spot size nonuniformities.
U.S. Pat. No. 4,712,930 to Maruno et al discloses a gradation thermal printhead and a gradation heat transfer printing apparatus which employs an energy controlling means for varying the voltage or pulse width of the signal pulse applied to a thermal printhead. The printing apparatus further has a power supply for the gradation thermal printhead and an energy controlling means for controlling the width of the pulse of the voltage applied to the thermal printhead in accordance with a recording signal.
U.S. Pat. No. 4,536,774 to Inui et al discloses a thermal head drive circuit which improves printing quality by using data from previously printed lines to compute a corrected pulse energy for the line being printed. A pulse energy operator uses data from a heat accumulation state operator, a memory which has data on the pulse energy used in the previously printed lines, and from either a pulse interval detector or a temperature detector.
U.S. Pat. No. 4,712,172 to Kiyohara et al discloses the use of the heating elements to preheat the printhead in the vicinity of the nozzles by subthreshold energy pulses insufficient to expel ink droplets to lower the viscosity of any plug of ink at the nozzles from which water has evaporated. Typically this preheating with subthreshold pulses is done when the ink jet printer is turned on or after it has sat idle for a period of time.
U.S. Pat. No. 4,791,435 to Smith et al discloses a thermal ink jet printhead having temperature sensors to provide the input needed to estimate the printhead temperature, so that the printhead may be kept at the desired predetermined time by slowing down the printing, if it is too hot to cool it off, or adds warming pulses too short to expel droplets, if it is too cold. All decisions and actions are made preceding a printing operation.
U.S. Pat. No. 4,910,528 to Firl et al discloses the use of a temperature sensor to measure the printhead temperature and a microcomputer to determine the pattern of droplets to be printed, so that prior to the commencement of printing, the number of droplets required to print the printed swath is known and used to predict the temperature at the end of swath. If the predicted printhead temperature exceeds a maximum value, the start of printing can be delayed or the printing mode can be modified. If the predicted printhead temperature is below a minimum value, the heating elements are pulsed with non-droplet ejecting current pulses or the sensor can be used as a supplementary heater to warmup the printhead before the start of printing. In conjunction with the current temperature of the printhead as sensed by a sensor thereon, the future printing demand is utilized to predict the printhead temperature at the end of the printing of a swath of information and the printing modified to ensure that the temperature limits are not exceeded.
U.S. Pat. No. 4,719,472 to Arakawa discloses the use of a separate heater and temperature sensor to heat and monitor the temperature of the ink in the reservoir to adjust the viscosity of the ink.
U.S. Pat. No. 4,490,728 to Vaught et al discloses the use of a two part electrical pulse to the heating elements of a thermal ink jet printer. The pulses comprise a precursor pulse insufficient to vaporize the ink following by a nucleation pulse to expel an ink droplet.